If you’ve run into an issue where you are trying to create a Parallels VM but the installation process is stuck on the grey startup screen with the Apple logo then chances are its down to the VM System Settings
All you need to do to remedy this is shut down the machine (Click the power icon in the bottom left and choose Shut Down)

Once it’s shut down you then click on the cog icon in the bottom right to get to the Config options and set them as follows

On the General tab change the CPUs: to 2

On the Hardware tab, click on the Video option, then increase the Video Memory to 512MB

and thats it

Restart the installation process and you should now find that Mountain Lion installs as you’d expect